So good, in fact, that seaweed might soon be an ingredient in functional foods - to make white bread, for example, higher in fibre. Scientists at the University of Newcastle upon Tyne have researched alginate, a substance in brown seaweed, and found that it can strengthen gut mucus (which protects the gut wall), slow down digestion (so you feel fuller for longer) and make food release its energy more slowly (ie, it is low-GI, and therefore good). It's also high in fibre. A study of the faecal flora of Japanese women (well, someone's got to do it) showed that high seaweed intake increases the good bacteria in the gut. The enzymes in kombu, which you can add in dried form to soups and stews, help pre-digest pulses, which in turn reduce wind.

High in nutrients, low in calories

You, too, may have thought, "But who can eat this stuff for breakfast?" when reading Japanese Women Don't Get Old Or Fat, by Naomi Moriyama and William Doyle (£12.99, Vermilion), but the judicious addition of nori or wakame to a bowl of noodle soup or stir-fry will tot up only 30 extra calories or so while packing in loads of mineral and trace elements. Nutritionist Ian Marber, of the Food Doctor clinic, says, "We don't farm the sea, so there will be sustained exposure to minerals" - in other words, there will be a level you might not get in vegetables grown in nutrient-poor soil. Arame and wakame are great sources of calcium, iodine, folate and magnesium, while purple laver is especially rich in B vitamins, according to a study reported in the British Journal Of Nutrition.

May improve heart health

Wakame has been shown to prevent high blood pressure in animals, according to a report in the Journal Of Nutrition. And research from Kyoto University showed that the fibres from brown seaweed lowered blood pressure and reduced the risk of stroke in animals predisposed to cardiovascular problems. But can we extrapolate from animal studies to sushi fans? A 25-year study of the longest-lived population, the Okinawans, who have unfurry arteries, low cholesterol and low homocysteine (a heart-damaging chemical) levels, showed that sea vegetables were among the seven to 10 portions of fruit and vegetables they eat daily. So they're part of a package deal in Okinawa, making sea vegetables a valuable addition to the much-touted variety of vegetables and fruit we are told to eat for health.

Has heavy-duty detox properties

Spa aficionados and cellulite sufferers may recall being swathed in the browny green stuff, but what happens when you eat it? A study from McGill University in Canada showed that seaweed was great for detoxing the body from the radioactive chemical strontium - admittedly, this is unlikely to be much of a problem unless you happen to live near a melted-down nuclear power plant (not for nothing did seaweed sales rocket in the Soviet Union post-Chernobyl). However, seaweed also mops up the toxins cadmium and lead, not only present in cigarette smoke but, says Dr David Santillo, senior science researcher at Greenpeace, is also in the environment from industry and transport. Make sure, however, that your seaweed comes from a reputable source, such as Clearspring (020-8749 1781, clearspring.co.uk) or, for Irish Moss drinks, Grace Foods (gracefoods.com).

May help regulate hormones

Seaweed is very high in lignans - these are plant substances that become phytoestrogens in the body, which help to block the chemical oestrogens that can predispose people to cancers such as breast cancer. Dr Jane Teas of Harvard University published a paper saying that kelp consumption might be a factor in the lower rates of breast cancer in Japan, and she is now researching the effects of seaweed as a natural replacement for HRT. Dr Kat Arney of Cancer Research UK points out that most studies have been conducted in the laboratory, but adds that "It's important to study whether sea vegetables can bring benefits, and we are currently investigating whether certain vegetables can protect against cancer."

An all-round tonic

In Ireland and the Caribbean, seaweed-based drinks and soups are drunk as a regular pick-me-up, or after an illness. Greg Lampert, director of the herbal course at the College of Integrated Chinese Medicine, says, "Kelp is used to reduce phlegm and soften hardness; it also promotes urination and reduces swelling." Others claim it has gastric qualities and acts as a hangover cure.

Goes well with other healthy foods

Let's face it, you're not going to have seaweed with a side of chips. With its strong, smoky flavour, you're hardly likely to binge on the stuff, either. But it goes well with sushi, tofu, miso soup, salads, vegetable stews and stir-fries, and plates of greens. It's probably the synergistic effects of all those healthy ingredients that keep Japanese women slim and the Okinawans living past 100, as well as giving people on macrobiotic diets (of which seaweed is a staple) lustrous hair and glowing complexions.